X-ray structural investigations are being carried out with the aim of studying a number of important problems. Work is being carried out on a number of molecular analogs of dinucleotides, where the sugar and phosphate linkages have been replaced by a three-carbon bridge, or by a hydroxy-tetrahydrofuran ring. These studies should cause light on the preferred conformations of dinucleotides and have important consequences on our knowledge of the detailed structure of nucleic acids. Structural studies are also being undertaken on some epsilon-modified nucleoside salts. X-ray analyses are also proposed on several key molecules - vernamycin (metal-free), and derivatives of the poly-oxygen antibiotic, X-537A -- which are involved in metal ion transport problems. A major object of this work is to discover the conformational changes that take place upon uptake of metal ions. These changes are being probed by a combination of X-ray crystallography and nmr techniques. The detailed structure of several polyene and oligopeptide antibiotics will also be probed by X-ray diffraction methods. In general, very little is known about these structures and combined X-ray diffraction and mass-spectrometric investigations offer the most efficient means of providing such information. Some preliminary studies are in progress regarding the structure of organic compounds that are obtained from marine sources and contain halogen.